1. Field of the Invention
The present invention relates to a plate for use as a light shielding blade that is required to move at a high speed, such as shutter blades of a focal plane shutter or a lens shutter of a camera, or diaphragm blades thereof.
b 2. Related Background Art
For such light shielding blades, there are required a small actuating force and a sufficient bending rigidity, in addition to the ability for high-speed movement.
For meeting these requirements, there is required a light shielding blade of light weight, high strength and high elasticity.
The present inventors has investigated a plastic sheet reinforced with continuous carbon fibers oriented in a predetermined direction (hereinafter referred to as CFRP (carbon fiber reinforced plastic) sheet), and identified that a plate material obtained by laminating a plurality of such sheets with mutually orthogonal or substantially orthogonal fiber directions was suitable as the material for light shielding blades capable of high speed movement in view of weight, strength and ease of manufacture (cf. Japanese Laid-open Patent No. 59-61827).
In this case, said plate material is produced by laminating a plurality of prepreg sheets which are precursors of CFRP sheets in such a manner that the directions of fibers mutually cross orthogonally or substantially orthogonally, and hardening the entire laminate by heating in a pressed state.
Said prepreg sheet is obtained by aligning continuous carbon fibers in a predetermined direction, in the form of a thin plate, impregnating said carbon fibers with thermosetting resin liquid which is a precursor of matrix resin (for example unhardened liquid of epoxy resin or unsaturated polyester) and transforming said resin liquid into so-called B-stage, in which the resin is already solidified and lacks fluidity but can be finally hardened by heating. It has a low resin content, and, in appearance, it looks like a bundle of hair-like carbon fibers each thinly coated with glue-like resin liquid.
Said plate material is worked (for example cutting and drilling) to obtain a predetermined shape of the light shielding blade. The light shielding blade thus obtained requires a low actuating force, has sufficient bending rigidity and can still be moved at a high speed.
The light shielding blade for high speed running has to be of a thickness not exceeding 0.12 mm and to be finished with sufficient flatness without bending. In addition, it has to maintain stable performance and quality even in varying circumferential conditions.
However, the conventionally known light shielding blades have been found to often show bending under a change in temperature and humidity, even if the flatness seems satisfactory in the beginning.
For this reason, if a shutter employing such light shielding blades is used in a camera, the bending of the blades may result in uneven blade running, fluctuations in the shutter speed, breakage of blades and light leakage through gaps between the blades.